Selective display system

ABSTRACT

A system for selectively displaying symbols contained on a display curtain involving the noncontiguous sensing of identifiers associated with each symbol and continuous comparison of sensed identifiers with a command identifier indicative of a symbol desired to be displayed, said comparison being performed by nonretentive parallel parity detection circuitry.

United States Patent Inventors Delmar G. Fields 82-51 Foxbay Road, Union Lake, Mich.

48082; Jon Eckert, 1633 Paseo De Roca, Hacienda Heights, Calif. 91745 Appl. No. 13,914

Filed Oct. 23, 1969 Patented Dec. 28, 1971 SELECTIVE DISPLAY SYSTEM 9 Claims, 4 Drawing Figs.

US. Cl 340/324 R, 40/31, 40/86 R, 235/61.1 l E, 307/218, 340/334 Int. Cl G08b 5/22 Field of Search 340/ 324,

325,173 LT, 324 R, 334; 235/61.l 15, 61.6 A, 61.1 1; 307/2l8;40/86, 31

[56] References Cited UNlTED STATES PATENTS 2,124,906 7/1938 Bryce 340/173 L 2,820,907 1/1958 Silverman... 235/6l.115X 3,031,585 4/1962 Frady 307/218 X 3,091,737 5/1963 Tellerman et a1 307/218 X 3,111,658 11/1963 Ehrat 340/325 3,178,699 4/1965 Burton 340/325 X 3,321,639 5/1967 Fowler et al. 307/218 X 3,386,088 5/1968 Johnston 340/324 Primary Examiner-David L. Trafton AnorneyWatson, Leavenworth & Kelton ABSTRACT: A system for selectively displaying symbols contained on a display curtain involving the noncontiguous sensing of identifiers associated with each symbol and continuous comparison of sensed identifiers with a command identifier indicative of a symbol desired to be displayed, said comparison being performed by nonretentive parallel parity detection circuitry.

Patented Dec. 28, 1971 3 Sheets-Sheet l Patented Dec. 28, 1971 3 Sheets-Sheet I w mt K i Q \ww III N IQ m x m UN M m Q Q m \ww IT. M iv 0% Q m L4 u J. W m m W W w @w m M Q R My VA, Q N QM R Ji 1 u .QNQ $2 QM Q\\ m M 1| L1|F u WW U m n 1 r 4 Q v MM Patented Dec. 28, 1971 3,631,454

3 Sheets-Sheet 5 SELECTIVE DISPLAY SYSTEM BACKGROUND OF THE INVENTION This invention relates to systems for displaying selected ones of a plurality of graphic symbols contained on a translatable curtain, and more particularly to systems of this type having high symbol capacity and low access time for display of a selected symbol.

Prior art selective symbol display systems are generally of two types distinguishable by the operational features of symbol capacity and access time, and by structural means employed therein for symbol location and presentation. Systems of both types are operatively responsive to operator or machine demand to place a selected symbol in registry with a presentation window. Similarly, both types of systems have in common the incorporation of sensible identifiers on the translatable curtain, in the form of distinct digital indicia patterns associated with each display symbol and exclusively indicative thereof, and means for continuously sensing the identifier of each symbol passing a given reference point. Beyond these common characteristics, these types of prior art systems are substantially different.

In one type of presently known system, employed in applications requiring high capacity but permitting extended access time, the graphic symbol identifiers, in the form of sets of curtain apertures, are sensed contiguously by contact fingers disposed on alternate sides of the curtain, opposed individual fingers forming a conductive circuit when juxtaposed with an identifier digit. The fingers are connected through nonelectronic, nonmemory circuitry to the curtain translator motor, such circuitry generally being in the form of a cumbersome wiring matrix and associated relays, switches, racks, etc., adapted to interrupt curtain translation upon correspondence of sensed identifier digits and selected symbol identifier digits. To provide high capacity, this type of system incorporates a dual-ended curtain transported from a feed roller to a takeup roller, the speed of curtain exchange therebetween being limited by the requirements of the contiguous identifier sensing means, and the durability of the curtain against wear. Whereas such systems incorporate curtain reversal features, identifier sensing is aptly performed in only one direction of curtain travel as will be discussed in detail hereinafter.

In the alternate type of prior art system, adapted for use in applications requiring low access time but permitting quite limited symbol capacity, high-speed curtain translation is imperative. Thus, in such systems, identifier sensing is performed in a noncontiguous manner by optical or magnetic means. Parity determination between sensed identifiers and the identifier of the selected symbol is required to be concomitantly fast, necessitating the use of electronic parity circuits in contrast to the wiring matrix and electromechanical means of the above-discussed systems. This high-speed system is presently characterized by serial parity detection circuitry, temporary storage circuitry and associated clocking circuitry both for identifier sensing and parity determination, and involves considerable expense in manufacture in comparison to the slower system. Thus, the faster system has generally been restricted to computer-associated usage for selective presentation of limited numeric or alphabetic characters contained on an endless translatable member. The slower system is found in various changeable exhibitors, destination indicators, mobile advertising displays, and generally in applications requiring less expensive equipment sufficiently rugged to be operable in mobile vehicles or other transient environments.

Whereas, in present day high-speed rail vehicles operating in crowded urban areas there is substantial need for a high capacity, high access rate selective display system, neither of the above systems is satisfactory. The first system, alone inappropriate on the basis of its extended access time, is further objectionable by reason of display member wear, apparatus volume and maintenance difficulties associated with said cumbersome electromechanical and wiring assembly for parity detection. The second system, entirely suitable in respect of its low access time, is of limited symbol capacity and of considerable expense. Moreover, by reason of its dependence on electronic memory, it is entirely unsuited to an operational environment wherein primary power interruptions, e.g. by reason of third rail discontinuities, are common.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a selective display system having extensive symbol capacity and low access time for display of a selected symbol.

It is a further object of the present invention to provide a high capacity, low access time selective display system adapted for use in applications susceptible to periodic power interruption.

It is an additional object of the present invention to provide a selective display system of the type having noncontiguous identifier-sensing means and electronic parity detection means not requiring temporary or permanent storage facility.

It is a further object of the invention to provide novel and simplified circuitry for use in parity detection.

It is an additional object of this invention to provide improved sensing means for selective display systems.

In the efficient attainment of these objects, there is provided in the present invention a selective display system comprising a translatable curtain containing a plurality of display symbols and associated identifiers in the form of binary sequences of sensible digits, a curtain drive motor, drive motor control means, an operator or machine-controlled symbol selector, noncontiguous symbol identifier-sensing means and nonretentive logic circuit means performing parallel parity detection on output signals of the symbol selector and the sensing means and directingoperation of said drive motor control means.

In a particularly preferred embodiment, adapted for use in destination indication for high speed rail vehicles, the display curtain is driven bidirectionally and the sensing means may be adapted to sense symbol identifiers such that motor dynamic braking error is divided equally for both directions of curtain travel.

The system logic circuit means preferably comprises a plurality of first EXCLUSIVE OR circuits each determining parity between respective digits of sensed identifiers and the identifier generated by said symbol selector. The output signal of each one of these circuits is applied to an OR circuit which directs said motor drive controller.

The above and other objects and features of the invention will be evident from the following detailed description of preferred embodiments of the invention illustrated in the accompanying drawings, wherein like numerals identify like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagrammatic illustration of the circuit elements of a selective display system constructed in accordance with the invention and display apparatus employed in conjunction therewith.

FIG. 2 is a detailed block diagrammatic illustration of certain of the functional blocks shown in FIG. 1.

FIG. 3 is a schematic diagram of preferred circuitry for use in parity detection.

FIG. 4 is an illustration of identifier-sensing means particularly suited for use in systems employing bidirectional display curtain translation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, selective display system 10 is contained in a housing having a front wall 12 defining a presentation window I4. Mounted rearwardly of wall 12 is a display curtain or translatable member 16 containing a plurality of graphic symbols l8 aligned thereon for registry with window 14 to permit viewing from the forward side of wall 12. Curtain 16 is dualended, one end being affixed to roller 20 and the other to roller 22. The rollers are connected respectively to shafts 24 and 26 which are complementarily driven by gear assembly 28.

In association with each symbol l8, curtain 16 includes a discrete identifier 3t), constituted by a digital pattern of sensible indicia 32. As illustrated, six identifier rows a through f are available thus permitting discrete identification of numerous symbols. As will be discussed below, most of such patterns are used for symbol identification. On pattern is reserved for control purposes.

While sensible indicia 32 may be magnetic or like active elements deposited or otherwise affixed to curtain to they are preferably formed integrally in the curtain, as by translucencies therein. Furthermore, where it is desired to employ all central curtain area for carrying display symbology, or where the symbology is itself translucent, indicia 32 may be disposed in lateral side areas of the curtain on one or both sides of the display symbology.

Fixedly supported to wall 12 below window 114 forwardly of curtain 16 is lamp assembly 34 continuously energized by power applied thereto from supply 36 over line 38. Disposed oppositely of lamp assembly 34 and rearwardly of curtain 16 is identifier sensing means 40. This assembly comprises six independent phototransistor units having sense fields each restricted to one of curtain rows a through f. Each sensor unit applies its output signal to one of the conductors 42a through 42f. As will be evident, digital pulse patterns corresponding to identifiers 30 will be generated on the conductor group 42 as curtain 16 is translated between rollers and 22, identifier indicia patterns being detectable by simultaneous examination of each conductor 42a through 42f. Curtain translation is provided upon excitation of motor 44, power being applied thereto from motor drive controller 46 over line 48. Motor output shaft 50 is coupled to gear assembly 28.

The selection of a particular one of display symbols I8 is effected through symbol selector 52, which is responsive to an operator or machine demand input to apply to each of conductors 54a through 4f a signal (DC level) or no signal (ground potential) depending upon the indicia pattern associated with the selected symbol. The digital pulse pattern corresponding to the indicia pattern is detectable by simultaneous examination of each conductor 54a through 54f, as in the case of conductor group 42 discussed above.

Selector 52, preferably remote from the remaining elements of the system of FIG. I as indicated by the dotted line 56, is separately powered at its location, and may be employed in common by numerous systems shown in FIG. l which may be disposed, for example, throughout a multicar rail vehicle. For instance, it may be desired to employ a single selector 52 at a location convenient to the conductor of the rail vehicle in a forward car. By control of the single selector, he may thus alter the destination displayed in each of the rail cars. This system arrangement is particularly appropriate for indication of stations in congested rural areas spaced apart by several minutes of vehicle travel.

It will be evident that selector 52 may take any number of forms without distinguishing its cooperation with the present system. It is an ancillary device adapted merely to continuously generate particular pulse patterns for different input demands. A key-operated matrix selectively connecting a bilevel DC power supply to conductors 540 through 54f is entirely suitable, as would be a serial chain of flip-flops set by pulse generator to assume an appropriate on-off configuration, each flip-flop being connected to one of the conductors 540 through 54f.

Logic circuit means 58 is connected both to conductor group 42 and conductor group 54 and comprises nonretentive circuitry throughout, i.e. no circuit elements having memory are employed therein. In achieving a first function, means 58 incorporates first circuitry adapted to detect parity or nonparity of input signals applied thereto by conductors 42a and 54a, 42b and 54b, 42c and 54c, 42d and 54d, 42:: and 54a, and 42f and 54f, and to generate a signal on output line 60 upon a condition of nonparity of any one of said input signal pairs. In

achieving a further function, necessary in applications wherein curtain 16 is dual-ended and is translated in either of two directions, means 58 incorporates second circuitry adapted to detect particular control pulse patterns presented by conductors 42 indicating that the ends of the curtain are being approached, and to thereupon generate a signal on output line 62. Preferred circuitry for performance of these functions of means 58 will be discussed in detail hereinafter in connection with FIG. 2.

By virtue of the use of nonretentive circuitry in means 58 for parallel parity determination, the system of the invention, while susceptible to power failure or temporary interruptions of power, is not prejudiced in operation thereby. Since the system has no memory of its activity preceding power interruption, it merely continues it normal searching for parity conditions. In contrast, systems of the type discussed above and employing electronic memory in the form of flip-flops, registers, etc., in performing serial parity determination, have no assurance of totally clearing prior memory in the absence of positive power-sensitive circuitry operative to clear the system following power interruption or fluctuation. Where certain memory elements are improperly set" in memory state upon and by reason of power fluctuation or failure, erroneous parity determination is possible. Same is in fact likely to occur in the referenced application wherein system power is dependent upon continuous physical contact between a third rail" and an appendage of the vehicle using the display system.

Lines 60 and 62 are connected to motor drive controller 46 which receives power from supply 36 over line 64. This unit is adapted to selectively apply power to output line 48 in response to signals on line 60, the polarity of the power thus applied to line 60 being reversed in response to signals on line 62. Thus, whenever the above-mentioned parity conditions are not present, motor-driving power is gated through the controller 46. In the dual-ended curtain application, this motordriving power is reversed by the controller as the end-of-curtain identifiers are sensed. Motor reverse drive will continue until parity conditions occur.

Referring to FIG. 2, wherein said preferred circuits for use in logic means 58 and for use in motor drive controller 46 are set forth, conductor groups 42 and 54 of FIG. I are shown at the left-hand side of the drawing, and lines 60 and 62 of FIG. l are shown centrally. Power lines 64a and 64b correspond to line 64 of FIG. 1 and motor excitation lines 48a and 48b correspond to line 48 of FIG. I. Internally of means 58 there is provided a plurality of logic circuits 66a through 66f, circuit 66a receiving input signals from lines 42a and 54a, circuit 66b receiving input signals from line 42b and 54b, etc. Circuits 66 have single output terminals which are connected to lines 680 through 68f, respectively. Each of these lines terminates at an input terminal of logic circuit 70, the output terminal of which is connected to line 64).

Since it is desired to provide a motor-drive-controller-energizing signal on line 6t) at all times other than that time at which the indicia signals generated by sensings means 40 are identical with the indicia signals generated by selector 52, circuits 66 are EXCLUSIVE OR circuits, i.e. they generate output signals only at times during which either of the input signals thereto are TRUE or ON. Since it is further desired to continue curtain translation until parity exists as between all input signal pairs, circuit 70 is an OR circuit. Thus, if five of the six indicia digit pairs are in correspondence, one of circuits 66 will generate an output signal and circuit 70 will respond thereto to insure that a motor drive controller energization signal will continue to be generated.

Circuits 662 and 66f are further connected by lines 72 and 74 respectively to the input terminals of AND-circuit 76. The single output terminal of this circuit is connected to line 62. Evidently, a signal will be generated on line 62 only when the indicia pattern sensed by unit 40 contains transparencies in both of rows e andf, such pattern being reserved exclusively for curtain reversal indication and disposed at both ends of the curtain. This assignment of indicia digits for curtain reversal indication is completely arbitrary, and where it is desired to maximize the number of total patterns which can be assigned to display symbol indicia, AND-circuit 76 may receive input signals from all of circuits 66a through 66f, whereby all but one of the total available indicia patterns is readily usable for symbol identification.

Motor drive controller 46 is comprised of two serially connected circuits 78 and 80. Circuit 78 is a power gating circuit operatively responsive to signals applied thereto from line 60 to connect line 64a and Mb to output lines 82a and 82b. An electronic switch of substantial power capacity, such as a silicon-controlled rectifier is preferably employed.

Lines 82a and 82b terminate at polarity-reversing circuit 80, a circuit operatively responsive to signals applied thereto from line 62 to reverse the manner of connection of lines 82a and 82b and output lines 48a and 48b, as shown graphically in FIG. 2. This circuit, upon the occurrence of a signal on line 62, maintains the manner of interconnection thereby effected until occurrence of the next reversal signal. A particularly preferred arrangement comprises a low-level monostable electronic switch with an associated bipolar power relay.

The operation of the system of FIGS. 1 and 2 will be evident from the following exemplary performance of the system in response to the generation by selector 52 of a pulse pattern other than that associated with the display symbol then in registry with window 14.

To fully illustrate the operation system of the system, it will be assumed that polarity reversing circuit 80 is in the position thereof which interconnects lines 82a and 821; with lines 48a and 48b such that curtain 16 will be translated upwardly, and that the selected display symbol has the associated indicia pattern 01 1 101, i.e. that immediately above window 114 in FIG. 1.

Since the indicia pattern in position before sensing means 40 assumed to be 01 l 100, does not correspond with that now generated by conductor group 54, at least one of the EXCLU- SIVE OR circuits 6-5 will generate an output signal on line 68. In the example, the sixth digits do not correspond, and circuit 66f will generate an output signal on line 68f. Such signal will be passed through OR-circuit 70 and will be applied over line 60 to circuit 78, whereupon motor energizing power will be switched therethrough and through circuit 80 over lines 48a and 48b to motor 44.

Under the assumed conditions, the selected display is translated away from rather than toward presentation window 14, and throughout such curtain translation, logic circuit means 58 continuously generates a signal on line 60, nonparity occurring continuously throughout such curtain translation. It

will be observed that nonparity will occur as curtain areas other than those containing identifiers are juxtaposed with sensing means 40 since the all-opaque identifier pattern, all digits FALSE, is not employed for any identifier.

As curtain 16 unravels fully from roller 22, the indicia pattern reserved for end of curtain indication will be sensed by means 40. At this instant, lines 72 and 74 are energized by v sensing means 40 and a signal will be generated on line 62 by AND-circuit 76. Circuit 80 will thereby be energized. With applied power thus reversed, motor 414 will reverse direction and roller 22 will be driven to collect curtain 16. .The selected display will now approach window 14, and when it is in registry therewith, sensing means 4% will apply to conductor group 42 precisely the same pulse pattern generated on conductor group 54. Thereupon, parity will be detected in each of circuits 66 and no input signals will be applied to circuit 70. Thus, no energizing signals will be applied over line to motor drive controller 46. Motor 44 is preferably dynamically braked upon discontinuance of energizing signals minimizing further translation of curtain 16.

A novel EXCLUSIVE 0R circuit for expeditious use in circuits 66 of FIG. 2 is illustrated in H0. 3. The circuit is provided with a pair of input terminals $611 and 86b and a single output terminal 88. A pair of transistors 90a and 90b are connected in series between the input terminals, each transistor havinga collector 92, an emitter 94 and a base as. Collector Terminal 86b Terminal 88 Terminal 860 It will be evident from the table that an exclusive or function is performed. An output signal will be provided at terminal 88 only where wither terminal 86a or terminal 86b receives an input signal and not where both receive input signals or neither receives input signals.

In FIG. 4, display curtain 16 is illustrated in cross section with associated lamp assembly 34 and sensing means 40, sensing means 40 being in a form particularly adapted for use in a system involving bidirectional travel of curtain 16. Assume, for the moment, that a single sensing element, i.e. of a phototransistor, is employed for sensing of identifier digits in each row of curtain 116, and that the system incorporates a predetermined maximum time delay between the sensing of identifiers and the cessation off travel of curtain 16. If the curtain is traveling upwardly and a selected one of symbols l8 approaches window 14, the sensing element may be offset below the window, to compensate for said delay, by a specified distance equal to the overshoot attending said delay. Following identifier sensing, the system will overshoot, but the symbol will register with the presentation window appropriately.

0n the other hand, if a single sensing element is employed and curtain 16 is reversed in its direction of translation, the sensing element having been set so as to provide precise presentation in the former direction of curtain translation, a compound error in registration of the selected display symbol and the presentation window will occur if it is attempted to sense identifiers during reverse curtain translation. This error will comprise approximately twice the overshoot of the system, occasioned by the offset of the sensing element for compensation of error in the former direction of curtain travel plus the total overshoot in the reverse direction. Thus, prior dual-ended curtain systems aptly perform sensing in only one direction of travel.

System overshoot is of course uncontrollable. Certain electrical delay, however minute, resides in the electrical circuitry associated with sensor 40 and substantial time delay is associated with dynamic braking of motor 44. However, equal distribution of the error over both curtain travel directions is possible, as will be seen in the compensatory sensor structure of FIG. 4. In FIG. 4, sensing means 40 comprises, in association with each identifier digit, a pair of sensing elements 104a and 10411 spaced apart along the direction of curtain travel by a distance d approximately equal to one-half the predetermined total system overshoot. Output lines 106a and 106b respectively of each of the sensing elements are connected in common to line 42a. Evidently, as the curtain l6 approaches sensing means 40 from one direction, one of the sensing elements 104 will be energized in advance of the other. Curtain overshoot measured from the axis of that sensing element will be identical with curtain overshoot accompanying identifier sensing when the curtain is translated in the reverse direction wherein the other sensing element will be first energized. By appropriate setting'of the sensor means central axis, system overshoot, while not entirely compensated in one or the other direction of curtain travel, is reduced by 50 percent in each direction and the troublesome double-overshoot occurring in one curtain travel direction in the absence of such sensing means is entirely avoided.

What is claimed is:

1. A system for selecfiVlydisplaying gone of a plurality of graphic symbols contained on a transportable display member, each of said symbols having associated therewith an identifier comprising a distinct pattern of sensible indicia, said system comprising:

a. motive means operatively responsive to a control signal to transport said display member;

b. sensing means for generating first pluralities of signals,

each signal plurality being indicative of the identifier indicia pattern associated with the then displayed symbol;

c. display symbol selector means continuously generating a second plurality of signals indicative of the identifier indicia pattern associated with a symbol selected for dis- P y;

d. first circuit means comprising a plurality of EXCLUSIVE OR circuits each receiving from said sensing means an individual signal of one of said first pluralities of signals and to said display member, energized power conductors, and switching means interconnecting said motor and said conductors, said switching means being rendered conductive by control signal.

6. The system claimed in claim wherein said display member is dual-ended and further includes control identifiers comprising a distinct pattern of sensible indicia adjacent the ends thereof, said sensing means generating third pluralities of signals upon sensing said control identifiers, said circuit means receiving said third pluralities of signals and generating a directional control signal in response thereto, said motive means further including bipolar connecting means exclusively connecting said switching means said motor in alternate first or second interconnection manners and operatively responfrom said selector means the corresponding individual signal of said second plurality of signals for detecting parity or nonparity of corresponding individual signals of said first and second signal pluralities; and

e. second circuit means connected to said first circuit means and generating said control signal continuously unit parity is simultaneously detected for all said corresponding individual signals.

2. The system claimed in claim 1 wherein said second circuit means comprises an OR circuit receiving signals generated by each said EXCLUSIVE OR circuit, said OR circuit generating said control signal.

3. The system claimed in claim 1 wherein said EXCLUSlVE OR circuits each comprise first and second input terminals receiving said individual and said corresponding individual signals respectively, first and second transistors having directly connected emitters, collectors resistively connected respectively to said first and second input terminals, and bases resistively connected respectively to said second and first input terminals, and an output terminal, first and second diodes connecting said collectors to said output terminal.

4. The system claimed in claim 1 further including means illuminating one side of said display member, said display member comprising an opaque sheet bearing said graphic symbols and including translucent portions defining said identifiers, said sensing means including photoelectric transducers disposed adjacent the opposite side of said display member.

5. The system claimed in claim 1 wherein said motive means comprises a dynamically braked motor operatively connected sive to said directional control signal to alternate said manner of interconnection.

7. A system for selectively displaying one of a plurality of graphic symbols contained on a transportable display member, each of said symbols having associated therewith a distinct sensible identifier, said system comprising:

a. bidirectional motive means operatively responsive to a control signal to transport said display member;

b. sensing means comprising first and second sensing elements spaced from one another in the direction of display member travel, each element generating a signal indicative of the identifier associated with the then displayed 5 mbol; c. isplay symbol selector means continuously generating a signal indicative of the identifier associated with a symbol selected for display; and

d. circuit means receiving and detecting correspondence or noncorrespondence of said signals generated by said selector means and said sensor elements, and generating said control signal continuously until said correspondence is detected.

8. The system claimed in claim 7 wherein said transportable member is dual-ended and further includes direction control identifiers adjacent the ends thereof, said sensor elements generating further signals upon sensing said control identifiers, said circuit means receiving said further signals and operatively responsive thereto to reverse the direction of transportation provided to said member by said motive means.

9. The system claimed in claim 7 wherein said sensing elements are spaced apart by a distance of approximately half the distance travelled by said transport member after said correspondence detection. 

1. A system for selectively displaying gone of a plurality of graphic symbols contained on a transportable display member, each of said symbols having associated therewith an identifier comprising a distinct pattern of sensible indicia, said system comprising: a. motive means operatively responsive to a control signal to transport said display member; b. sensing means for generating first pluralities of signals, each signal plurality being indicative of the identifier indicia pattern associated with the then displayed symbol; c. display symbol selector means continuously generating a second plurality of signals indicative of the identifier indicia pattern associated with a symbol selected for display; d. first circuit means comprising a plurality of EXCLUSIVE OR circuits each receiving from said sensing means an individual signal of one of said first pluralities of signals and from said selector means the corresponding individual signal of said second plurality of signals for detecting parity or nonparity of corresponding individual signals of said first and second signal pluralities; and e. second circuit means connected to said first circuit means and generating said control signal continuously unit parity is simultaneously detected for all said corresponding individual signals.
 2. The system claimed in claim 1 wherein said second circuit means comprises an OR circuit receiving signals generated by each said EXCLUSIVE OR circuit, said OR circuit generating said control signal.
 3. The system claimed in claim 1 wherein said EXCLUSIVE OR circuits each comprise first and second input terminals receiving said individual and said corresponding individual signals respectively, first and second transistors having directly connected emitters, collectors resistively connected respectively to said first and second input terminals, and bases resistively connected respectively to said second and first input terminals, and an output terminal, first and second diodes connecting said collectors to said output terminal.
 4. The system claimed in claim 1 further including means illuminating one side of said display member, said display member comprising an opaque sheet bearing said graphic symbols and including translucent portions defining said identifiers, said sensing means including photoelectric transducers disposed adjacent the opposite side of said display member.
 5. The system claimed in claim 1 wherein said motive means comprises a dynamically braked motor operatively connected to said display member, energized power conductors, and switching means interconnecting said motor and said conductors, said switching means being rendered conductive by control signal.
 6. The system claimed in claim 5 wherein said display member is dual-ended and further includes control identifiers comprising a distinct pattern of sensible indicia adjacent the ends thereof, said sensing means generating third pluralities of signals upon sensing said control identifiers, said circuit means receiving said third pluralities of signals and generating a directional control signal in response thereto, said motive means further including bipolar connecting means exclusively connecting said switching means said motor in alternate first or second interconnection manners and operatively responsive to said directional control signal to alternate said manner of interconnection.
 7. A system for selectively displaying one of a plurality of graphic symbols contained on a transportable display member, each of said symbols having associated therewith a distinct sensible identifier, said system comprising: a. bidirectional motive means operatively responsive to a control signal to transport said display member; b. sensing means comprising first and second sensing elements spaced from one another in the direction of display member travel, each element generating a signal indicative of the identifier associated with the then displayed symbol; c. display symbol selector means continuously generating a signal indicative of the identifier associated with a symbol selected for display; and d. circuit means receiving and detecting correspondence or noncorrespondence of said signals generated by said selector means and said sensor elements, and generating said control signal continuously until said correspondence is detected.
 8. The system claimed in claim 7 wherein said transportable member is dual-ended and further includes direction control identifiers adjacent the ends thereof, said sensor elements generating further signals upon sensing said control identifiers, said circuit means receiving said further signals and operatively responsive thereto to reverse the direction of transportation provided to said member by said motive means.
 9. The system claimed in claim 7 wherein said sensing elements are spaced apart by a distance of approximately half the distance travelled by said transport member after saiD correspondence detection. 